Defluorination of wet process phosphoric acid

ABSTRACT

A process for the defluorination of phosphoric acid containing dissolved aluminum is described.

United StatES Patent 1 1 1111 3,718,729 Amin et al. 1 1 Feb. 27, 1973[54] DEFLUQRINATION OF WET PROCESS [56] References Cited PHOSPHORIC ACIDUNITED STATES PATENTS [75] Inventors: Ashok Babubhai Amin, Trenton, 2987 376 6/196] G 1 23/165 oss Lee Bmmw Lakeland 2,933,372 4/1960 Manning..23/165 F 3,151,941 10/1964 Hollingsworth mu. ..23/165 Assignee:American Cyanamid p y, 3,l93,35 l 7/l965 Mlllr 6! 3i. ..23/165 stmford1Conn FOREIGN PATENTS OR APPLICATIONS [22] I Filed: March 15, 1971 145'6,263 9/l966 Fran e 21 N I 1 Appl 0 124,099 Primary Examiner-Oscar R.Vertiz Assistant ExaminerGregory A. Heller 1521 US. c1 ..423/321 Arw y-Raymond 51 int. c1.....; ..C01b 25/16 [58] 57 ABSTRACT Field of Search..23/l65, 165 B A process for the defluorination of phosphoric acidcontaining dissolved aluminum is described.

11 Claims, 12 Drawing Figures PATENTEDFEBZYIQB 3,718,729

SHEET 01 0F 12 fIE. I

INVENTORS ASHOK BABUBHA/ AMI/V ROBERT LEE BR/STOW ATTORNEYPATENTEUFEBZTIQYS v 3,718,729

sum 03 0F 12 /.0 09 EFFECT OF 4/ 0 CONCENTRA r/o/v a/v DEFLUORl/VAT/O/VRATE Ar 85% u [62 o A 2/6 PERCENT FL UOR/NE l l l O 2 4 6 8 l0 l2 /4TIME f, HOURS INVENTORS ASHOK BA BUBHA/ A MIN ROBERT L 55 BR/S TOW BYW@WA TTOR/VE Y PATENTED FEB 2 71975 PERCENT FLUOR/NE SHEET OHUF 12DEFLUOR/NAT/O/V AT 85C WITH l 1 I I l TIME f, HOURS fIE. 4

' INVENTORS ASHOK BABUBHA/ AM/N ROBERT LEE BRISTOW BY MNW A 7' TORNE YPATENIED FEB 2 7197s PERCENT ELUOR/NE DEEL UORl/VA T/DN DATAEXPERIMENTAL DA TA 0 MODEL DATA T/ME, HOURS NVENTORS.

1 ASHOK BABUBHA/ AM/N ROBERT LEE BR/STOW WQW ATTORNEY PATENTEDFEBZYIQYES3,718,729

SHEET USBF 12 PERCENT Si 5/ CONCENTRATION o/sm/aur/o/v EXPERIMENTAL 0ATA 0 =MO0EL DA TA TEMP. 85C

I l I I 2 3 4 5 6 T/ME, I, HOURS INVENTORS. ASHOK BABUBHA/ AMI/V ROBERTLEE BRISTOW ATTORNEY PERCE/V 7' F L UOR/IVE PATENIED FEBZT 1975 sum IUUF12 DEFLUORI/VAT/O/V AT 0.5 L/TER/M/IV.

SPARE/N6 RATE 0 EXPERIMENTAL vnwss o CALCULATED VALUES TEMPERATURE ms cTURBINE spa-0 360 RPM 7'/ME, f HOURS INVENTORS. ASHOK BA BUBHA/ AMI/VROBERT LEE BR/STOW ATTORNEY fIE. [Z7

PERCENT 3/ PATENTED 3718,7253

SHEET 11 [1F 12 5/" CONCENTRATION DISTRIBUTION 0 l4 0 EXPER/MENTALVALUES 0 =CALCULA TED VALUES STRIPPl/VG RAT/0 3.334

TURBINE SPEED 360 RPM TEMPERATURE /05c U U 0 l l l TIME, I HOURSINVENTORS. ASHOK BABUBHA/ AM/A/ ROBE/PT LEE BR/STUW BYWQW ATTORNEYPATENTEDFEBZTIQYS 3718'729 snzjl'I 12 0F 1 l 4 5 6 7 a 9 /0 '2', PERHOUR INVENTORS. ASHOK BABUBHA/ AM/N ROBERT LEE BR/STOW BYWSW ATTORNEYDEFL UORINATION OF WET PROCESS PIIOSIIIORIC ACID The present inventionconcerns a process for defluorinating wet process phosphoric acid whichcontains dissolved aluminum.

BACKGROUND OF THE INVENTION There has developed in recent years asubstantial need for phosphates which are suitable for use in preparingmaterials such as calcium phosphates to be used as animal feedsupplements. Unfortunately, the phosphate rock from which suchphosphoric acid is prepared contains significant amounts of fluorine.For the manufacture of high quality plant fertilizers and animal foodsupplements, the fluorine level must be reduced to contain less than 1part by weight of fluorine per 100 parts by weight of phosphorus.Accordingly, numerous processes have been developed in an effort toproduce defluorinated wet process phosphoric acid in a convenient andeconomic manner.

One method which has been devised to effect defluorination of wetprocess phosphoric acid concerns the addition of silica to boilingphosphoric acid whereby the fluorides are removed as SiF vapor. Severalproblems have been encountered by such a process. The use of hightemperatures such as are encountered in boiling the wet processphosphoric acid necessitates the use of expensive corrosive resistantequipment as well as substantial energy. Furthermore, it has been foundthat when one attempts to reduce the fluorine concentration in the wetprocess acid prepared from materials such as the Florida phosphate rock,defluorination subsists at a fluorine concentration of about 0.4percent.

Accordingly, it is an object of the present invention to overcome one ormore of the above problems. These and other objects will become apparentfrom th description and examples which follow. I

THE PRESENT INVENTION It has been found that defluorination of wetprocess phosphoric acid containing undesirable amounts of fluorine canbe achieved by admixing the acid with an amount of silica at least aboutstoichiometrically equivalent to the fluorine to be removed therefromand heating the silica acid mixture in the range of from about 75 C. toabout l05 C. but below the boiling point of the mixture. Removal offluorine as SiF, is then achieved by establishing a gas-liquid interfacewith the heated mixture wherein said interface is sufficient toestablish a stripping ratio of from about 0.5 to about 20. The vaporizedSil is thereafter removed from the presence of the acid mixture whilemaintaining its tem perature above the condensation temperature of SiF,.

It has been found that where wet process phosphoric acid as preparedfromphosphate rock which contains substantial amounts of aluminum, thefluorine contained in the phosphoric acid is present as aluminumfluoride complexes such as All and All ions. It is believed'to be theexistence of such complexes which causes the removal of silicontetrafluoride from acid generated from Florida phosphate rock sodifficult in comparison with that prepared from substantially aluminumfree European rock.

The reaction scheme which prevails where silica is added to wet processphosphoric acid containing from about 0.8 percent to about 2.2 percentor higher by weight of dissolved aluminum can be expressed by thefollowing reaction scheme:

ZAIF, an 2 2A1+ 2H,F, l.

2H,F sio 2 5m, 2H,0 2.

Sir, 2A1+ .2 2A1F,+ sio 411 3.

The overall forward reaction is expressed by the reaction of l and (2)above. The overall reverse reaction is expressed by reaction (3) above.It can be seen from the equations above that where the concentration ofdissolved SiF (probably existing as SiF increases, the equilibriumexpressed in reaction (3) is shifted to the right favoring the formationof aluminum complexes which precludes the removal of fluorine byvolatilization. This is expressed by the following generic equation:

(ix/d1 k [F] k [Al X Equation I wherein X is the percent of Si in theacid, dX/dt is the rate of change in X due to the reactions above, k,and k; are the overall rate constants for the forward and reversereactions, respectively, F is the concentration of total fluorine in theacid at Timet and Al is the concentration of free aluminum ions (otherthan AlF-t).

It is apparent from equation I above that where the concentration ofdissolved silicon, denoted by X, is permitted to increase duringthecourse of the defluorination, the rate of defluorination will decreaseproportionally. This is experimentally manifested by conductingdefluorinations with various concentrations of dissolved silicon. Suchexperiments show that satisfactory defluorination is a function ofdissolved silicon concentration. Equation I also shows the detrimentalaffect of high concentrations of aluminum in the acid. The adverseeffect of such concentrations is minimized in the practice of thepresent invention by maintaining the dissolved silicon concentration, X,at a very low level, whereby the total value of the term k [Al X isminimized.

Since dissolved silica concentration is proportional to the totalfluorine concentration in the acid at any flxed rate of stripping, thepreferred level of silicon concentration is expressed by the ratio: (wt.%F/wt. %Si) Since dissolved Si concentration is inversely proportionalto the degree of stripping, there is a preferred or critical degree ofstripping which corresponds to the preferred or critical range ofsilicon concentration. The degree of stripping is indicated by theratio; mn/M (called the stripping ratio herein). In this expression, Mis the total amount of a batch acid to be defluorinated; m is the amountof acid which is being contacted with the inert gas for stripping perhour; and, n is the fraction of dissolved silicon which is removed asSiF, by stripping.

In the practice of the present invention, the SiF stripping can beconveniently achieved by one of three methods.

The first method of the present invention involves the mass transfer ofacid by spraying into a current of inert gas. This can be achieved, forexample, by

equipping a vessel containing M gallons of acid to be defluorinated witha means for recycling m gallons of acid per hour through a stream ofinert gas, wherein the recycled acid is in the form of a plurality ofsprays of sufficient number and size to establish the previouslyindicated stripping ratio. This can be achieved by an apparatus such asthat described in FIG. 2 of the attached drawings.

The second process involves the use of sparging with a current of inertgas whereby the gas bubbles are sufficiently small size to establish thepreviously indicated stripping ratio. This can be achieved byintroducing a stream of gas at the base of a high speed turbine bladestirrer such as that described in FIG. 1 of the attached drawings. Inthis case, M is the total volume of acid to be defluorinated; m is thevolume being contacted with air per hour; and n is the fraction ofdissolved silicon removed from the acid m by stripping. It is obviousthat the values of n can range from O to 1 depending on the acid/gasinterface area which has been created.

The third process which can be employed in the practice of the presentinvention employs a combination of sparging with an inert gas and anacid spray recycle as described in methods one and two above.

The instantaneous defluorination rate is related to the dissolvedsilicon concentration and the stripping ratio. In any given shortinterval of time, the amount of fluorine which is transferred to theinert gas in a recycle spraying process such as that described above,must be equal to the amount of fluorine lost by the acid. This can beexpressed by the following equations:

M -dF/dt=mn (2.71 X) EquationZ (dF/dt)/2.7l X= mn/M Equation 3 whereindF/dt the rate of defluorination; X the dissolved silicon concentration;2.71 the weight ratio of fluorine to silicon in SiF and, mn/M thestripping ratio.

Equation 3 is a general expression which can be used in the analysis ofboth the spraying, sparging and spraying-sparging methods describedabove. In the sparging type procedures, where it is not physicallypossible to measure the stripping ratio, it can be determined by the useof these equations, by measuring the rate of defluorination anddissolved silicon concentration at any given time.

The defluorination of wet process phosphoric acid containing highconcentrations of aluminum can further be described by Equation 4 below.This equation is derived by means of the kinetic Equation 1 above andthe material balances of the process. The equation is as follows:

dF/dt (2.71 mn)/M k,F (k [Al (mn/M) Equation 4 The concentration ofdissolved silicon can be expressed using a derivation of Equation 2above:

X=- (M/2.7l mn) dF/dt Equation 5 The values of the forward reaction rateconstants for different Al,0 concentrations and temperatures can befound experimentally by observing defluorination kinetics of smallbatches of acid in the laboratory under conditions of extremely highstripping ratios, in which case the dissolved silicon concentration inthe acid approaches O and the kinetic expression becomes:

' (dF/dt) k F Equation 6 The value of the reverse reaction rate constantk; is found by measuring dF/dt, d F/dt and X at any given time from thestart of the defluorination and then employing this data in Equations 3and 4 above. The data obtained by experimental defluorination andcomputer simulated runs is in very close agreement.

The preferred quantity of silica employed in the practice of the presentinvention is from about 0.8 to about 2.0 pounds of silica per pound offluorine contained in the acid to be defluorinated. It is also preferredto employ silica having a surface area from about 10 to about 500 squaremeters per gram. It is generally preferred to employ diatomaceous silicaor spraydried silica gel as the silica source.

The preferred inert gas is air; however, other inert gases such ascarbon dioxide, nitrogen, argon and the like may be employed in lieuthereof or in combination therewith.

The processes of the present invention are further described byreference to the figures herewith.

FIG. 1 is a diagrammatic representation of a sparging apparatus suitablefor use in the defluorination of the present invention.

FIG. 2 is a diagrammatic representation of a spraying apparatus which issuitable for use in the practice of the present invention. A suitableapparatus suitable for conducting the spray-sparge process of thepresent invention may be envisioned as an embodiment of both thespraying elements of FIG. 2 and the sparging elements of FIG. 1.

Now referring to FIG. 1, it can be seen that the defluorination iseffected in an insulated reactor 1 adapted to receive a stream ofstripping air through conduit 2 which is optionally heated by means ofthe heat exchanger 3. The conduit is adapted to discharge the air streambeneath the motor driven turbine blade agitator 4 whereby said airstream is dispersed throughout the wet process phosphoric acid containedin the reactor in the form of minute air bubbles. The reactor is furtherequipped with a valved air port 5 to allow air to be drawn through thereactor above the acid surface to strip the dispersed acid droplets andan exit port 7 to permit withdrawal from the reactor of the fluorinecontaining gases which have been stripped from said acid droplets.Withdrawal is achieved by actuation of the blower 9. The reactor isfurther equipped with insulation 8 to aid in the maintaining of thephosphoric acid temperature and heating means, such as, tracing 6, inthe reactor dome and exhaust conduit to permit heating of the vesselwalls in contact with the fluorine containing gases to a temperatureabove the condensation temperature or dew point of said gases. Thereactor is also equipped with a means for circulating the acid mixtureto facilitate the mixing of the acid, silica and air and to permit themaintenance of proper temperature for the acid defluorination process.To effect this circulation of acid, a conduit 16 having its inletorifice disposed in the bottom of the reactor is equipped with acirculation pump 17 of such capacity to provide a recirculation rate ofat least about 3 percent per minute, and preferably, between about 3percent and percent per minute of the total acid mixture undertreatment. As indicated, conduit 16 is equipped with a valve adapted topermit the flow of the acid through heat exchanger 19 whereby thetemperature of the recirculating acid may be adjusted as desired priorto the reintroduction of the acid to the reactor via conduit 20. Thevalve 18 is further adapted to permit withdrawal of the defluorinatedacid through conduit 21 into storage tank 22 from which it may bewithdrawn through conduit 23 by opening valve 24. As previouslyindicated, removal of the fluorine-containing gases is effected byactuation of blower 9. Also, said blower may be vented into theatmosphere without disruption of the process of the present invention.It is preferred to exhaust the fluorine-containing gases into ascrubbing tower 10 to prevent atmosphere contamination and to recoverthe fluorine-containing compounds. This is achieved by introducing saidgases into the water shower produced by spraying means 11 whereby thefluorine-containing gases are extracted from the exit gases to form anaqueous solution of fluorosilicic acid which is discharged through thebottom of the scrubbing tower through conduit 13 into a acid-storagevessel 14 from which it may be extracted as desired through conduit 15.The scrubbed exit gases are removed through conduit 12.

In operation of the apparatus of FIG. 1, it is generally preferred tocharge about 1.1 pound or a slight stoichiometric excess of silica perpound of fluorine contained in the acid to be defluorinated.Diatomaceous silica or spray-dried silica gel having a surface areabetweenabout l0 and about 500 square meters per gram and preferablybetween 10 and square meters per gram is employed as preferred silicasources. The silica may be charged through port 5 or other suitableconduit (not shown). The wet process phosphoric acid, which is alsocharged into the vessel through port 5 or other suitable conduit (notshown) has a P 0 content between about percent and 62 percent with apreferred range of from about percent to about 54 percent with afluorine content of from about 0.3 percent to about 3.0 percent. Thepercent of dissolved aluminum, generally expressed as aluminum oxide,will typically be in the range of from about 0.8 percent to about 2.2percent. In each case, the percents indicated are by weight. The acidfeed is generally maintained at a temperature of from about 75 C. toabout 105 C. but below the boiling point of said acid. It is generallypreferred to employ a sparging air stream having a temperature ofbetween about 24 C. and about 105 C. It is generally preferred to employthe sparging gas at a rate of from about 1 to about 5 cubic feet of airper minute per ton of acid to be defluorinated; however, depending onsuch factors as the acid impurities, the P,O, aluminum and fluorinecontent, the temperature, degree of agitation, silica concentration andits solubility in the acid, as little as about 0.5 or as much as about10.0 cubic feet of air per minute per ton of acid may be required toestablish the indicated stripping rate for the defluorination of thepresent invention.

Where operation of the apparatus depicted in attached FIG. 1 results inthe formation of excessive foam at the gas-liquid interface, shut downof the reactor can generally be avoided by directing a stream ofrecycled acid from conduit 20 onto the acid surface.

An unexpected degree of inhibition of the defluorination is caused byfailure to maintain the interior surfaces of the reactor dome attemperatures above the dew point of the effluent gas, i.e., above aboutC. Where insufficiently high temperatures are achieved, the SiF,condenses on the reactor walls where it decomposes to SiO and HF byreaction with water vapor. Heating of the reactor dome may be achievedby any convenient means as, for example, by electrical heating elementsor by passage of hot gases through conduits provided within the reactorwalls.

FIG. 2 illustrates another type of air-acid contacting vessel which maybe utilized in the defluorination process of the present invention. Inthis apparatus, the wet process phosphoric acid to be defluorinated ischarged through inlet 28 into holding tank 29. The acid is thendelivered to the insulated reactor 25 through conduit 30 by gravity orpump means herein not shown.

When the reactor is filled to the desired level, acid delivery is ceasedand agitation of the acid is begun. This in accomplished by actuatingmotor driven stirrer 31. To obtain satisfactory defluorination of theacid, the impeller of the stirrer will generally be rotated at a ratesufficient to produce an impeller tip speed of between about 2 and about25 feet per second (fps) and preferably between about 8 and about 15 fpswhen the impeller diameter to tank diameter ratio is more than 0.2.Higher tip speeds are required if the ratio is less than 0.2. Acid iswithdrawn from reactor 25 by actuating centrifugal pump 43 in thereactor exit line 42. The acid is thereby introduced through conduit 45into heat exchanger 36 which is maintained at a temperature of betweenabout 75 C. to about 121 C. to adjust the acid temperature from about 75C. to about C. The heated acid is then discharged onto the acid surfaceby way of conduit 35 which feeds the spray nozzles 32. Acid circulationrate is approximately 7 percent to 10 percent per minute of total acidtonage being treated. Shorter defluorination time can be achieved byincreasing the recycle rate. Recycle rates below 3 percent result inimpractical high defluorination times.

Diatomaceous silica or spray dried silica gel, having a surface area ofabout 15 to about 30 sq. m/gram for the diatomaceous silica or about 320to about 500 sq. mlgrarn for the silica gel is introduced throughconduit 53 into vessel 52 which is equipped with a motor driven agitator51. The vessel is then charged with phosphoric acid through conduit 50connected to the acid recirculating line. The acid/silica mixture ispumped into reactor 25 either via conduit 49 through circulating pump46, conduit 47, holding tank 29 and conduit 30 or through the circuitwhich includes conduit 45 and circulating pump 43, etc.

Silicon tetrafluoride is released from the acid in reactor 25 and themixing vessel 52. It is then withdrawn from said vessels through theinsulated and traced conduits 33, 55 to the gas scrubber inlet 56. Theinsulation and tracing prevent decomposition of SiF, on the walls of theexhaust conduit and refluxing of the fluorine into the acid. In thepresent system, approximately 2 to 16 cubic feet of air per minute perton of acid is utilized for the air stripping. If the specific velocityof stripping gas drawn through the system is excessive acid mist isentrapped in the effluent gas and the fluorosilicic acid obtained byaqueous scrubbing of the gases is contaminated and valueless. Generally,about 20 cubic feet per minute per ton of acid is the maximum flow whichcan be used in the present process without entraining acid mist with siFgases. SiF, vapors are passed through a scrubbing tower 57 with airscrubbed with an aqueous solution of fluorosilicic acid introducedthrough conduit 58 and sprayed over the gas stream by means of nozzles59. The sprayed acid is recirculated from tower 57 via conduit 62 andholding tank 63, conduit 64, recirculating pump 65. The scrubbed exhaustgases exit through conduit 60 through port 61 into a second scrubbingtower 67. Therein, the gases are scrubbed with a stream of fresh waterwhich enter the tower via conduit 68 and is sprayed over the gases withnozzles 69. The rescrubbed gases exit through conduit 70 with theassistance of exhaust fan 71, whereafter the purified exhaust gases areadmitted into the atmosphere via exhaust port 72. The fluorosilicic acidscrubbing solution from the second tower is returned to holding tank 63via conduit 73 whereafter it is recirculated through the first scrubbingtower 57. When the fluorosilicic acid concentration in the scrubbingunit reaches about 20 to about 25 percent, it is withdrawn from thesystem through conduit 66 by opening a valve therein (not shown). Theacid withdrawn therefrom may be marketed as such or subject to furthertreatment.

Reaction temperature of the wet process phosphoric acid in vessel 25 ismaintained by insulation 26 up to acid level. Decomposition of the SiFis prevented by the heating means 27 such as electric tracing orconduits within the wall through which hot fluids or gases are passed inthe reactor dome and gas exit lines. In the mixing vessel 52 heatingmeans 54 are only required in the dome to prevent condensation orrefluxing of the SiF, formed. The air inlet of the reaction vessel 74may optionally be equipped with a heating means (unshown) to assist inmaintaining the reaction temperature. As previously stated, the contentsof the reaction vessel are primarily heated by the heat exchange 36.This can be done by passing steam through conduit 37 and valve 39 andout a steam exit conduit 38. Control of the steam flow and thus thereaction temperature can be automatically achieved by means of atemperature recording controlling device 40. This device can bepositioned in conduit 34 which is adapted within the reaction vessel 25to optionally permit the introduction of steam or an air stream into thereaction vessel by opening valve 39 and/or other valve means (unshown).The defluorinated acid can be supplied to a car loading station orstorage vessel 48 by actuating pump 46 and opening a valve means(unshown) into vessel 48.

The present invention is further described by the following examples,which are not to be taken as limitative thereof. The percentages andparts in the above description and the following examples are in eachcase by weight, unless otherwise indicated.

EXAMPLE 1 A 4,000 gram sample of wet process phosphoric acid containing1.46% A1 and 0.85% F was defluorinated at 85 C. in an air spargedvessel, fitted with a 2 inch diameter turbine rotating at a speed of 350RPM. A stoichiometric amount of diatomaceous earth was added. The airspraying rate was 0.5 liters per minute.

Defluorination kinetic data was collected as shown in Table l. Theforward reaction rate constant k was obtained from the defluorinationkinetics of a separate run in which an extremely high stripping ratiowas used. From the data of Table I, the values of dF/dt and d werecalculated at time t= 3 hours.

Using Equations 3 and 4, the values of the stripping ratio mn/M and thereverse reaction rate constant k;, were then calculated to be 1.54 and12.2, respectively.

A simulated defluorination run was then carried out using an analogcomputer and the stripping ratio values and reverse rate constantobtained above. The comparison of the kinetic data from the experimentaland simulated runs are shown in FIGS. 8 and 9, respective- TABLE I Time,hrs. Fluorine Si t F X 0.0 0.85 0 0.5 0.76 1.0 0.63 0.0467 1.5 0.580.0373 2.0 0.49 0.028 3.0 0.39 0.0186 4.0 0.32 0.014 5.0 0.30 0.0093 6.00.26 0.00467 8.0 0.20

EXAMPLE 2 A defluorination run similar to the above was carried out at105 C. using a wet process acid containing 1.48% A1 0 and 1.03% F. Fromthe defluorination kinetic data dF/dt and a! at time t 1 hour werecalculated. The value of k, 1.45 at 105 C. was obtained from aseparately carried out defluorination run in which an extremely highstripping ratio was employed. From these data, the values of k;, andmn/M were calculated to be 468 and 3.334, respectively.

A simulated run was carried out using the above values of k and mn/M.The results of experimental and simulated defluorinations are shown inFIGS. 10 and l 1. The data are in high accord.

A number of other simulated defluorination runs were carried out toinvestigate the effect of different stripping ratios at C. and C. Fromthe results obtained, FIG. 12 was plotted. This plot shows thecriticality of the stripping ratio and the ratio of F /Si in the acid,on the defluorination time.

EXAMPLE 3 FIG. 3 shows the defluorination kinetic data from several runsin which wet process phosphoric acid containing various A1 0concentrations was defluorinated at a stripping ratio of about 6 and atemperature of about 85 C.

The data clearly shows the retarding effect of dissolved aluminumconcentration on defluorination rates. It also indicates that to achievedefluorination in same time period for acids of different A1 0 content,an acid having higher dissolved aluminum concentration would require useof a higher stripping ratio.

EXAMPLE4 FIG. 4 shows three simulated defluorinations carried out withthe use of the analog computer as previously described for the acidcompositions and operating conditions of Example 1, except that thestripping ratios were changed. Run A was carried out at a strippingratio of 0.15; Run B at a stripping ratio of 0.5; and, Run C at astripping ratio of 15.43. The defluorination times required to reducethe fluorine concentrations in the acids to 0.2 percent in Run C and RunB were 3.6 hours and 19 hours, respectively; whereas, the fluorineconcentration of the acid at 20 hours time in Run A was still only0.35percent.

The dissolved silicon concentrations in the acid for Run A and Run C areshown in FIG. 5. These results again show the importance of maintaininglow silicon concentrations in the acid for obtaining high defluorinationrates.

EXAMPLE 5 This batch defluorination run using the process of the presentinvention was carried out to show the importance of dissolved siliconconcentration levels on defluorination rates at two different strippingratios. During the first 1.5 hours of the run, high stripping ratio of25 was used. In the rest of the batch run, the stripping ratio wasdecreased to 0.3 by reducing the agitator speed from 1,800 RPM to 60RPM.

In each case, the defluorination was carried out on a 4,000 gm. acidsample employing a 0.5 liter/min. air rate and a reaction temperature of85 C. The A1 concentration was 1.46 percent.

The results achieved are set forth in FIG. 6, which graphically showshow the decrease in rate of fluorine removal and increase inconcentration of dissolved silicon is effected.

EXAMPLE 6 Experimental batch defluorination of 4,000 gms. of wet processacid carried out at 105 C. and different RPMs of a 2 inch diameterturbine and an air sparging of 0.5 liter/min. was carried out by theprocess of the present invention.

Stripping ratios were calculated with the use of Equations 3 and 4.

The effect of variations in stripping ratios achieved by varying thegas-liquid interface via turbine speed is graphically shown by the plotof data obtained, set forth in FIG. 7.

EXAMPLE 7 The following tests demonstrate the necessity for providingthe reactor dome, etc. with an additional heating means to avoidcondensation and decomposition of SiF on the surfacesthereof.

In these tests acid having a P 0 content of about 51 percent andcontaining 1.77 percent fluorine was placed in a flask and mixed withdiatomaceous silica in an amount stoichiometrically equivalent to thefluorine present. Air was bubbled through the agitated acid at a rate of1 liter/minute/l ,000 gm. of acid and the mixture was maintained at C..-:7 C. during treatment. Data obtained show that after about elevenhours fluorine reduction is essentially halted even though 0.57% Si0 isstill present in the acid. The results are set forth in Table 11 below.

TABLE II Defluorination by Air Sparging without Heating Mantle P,O F,P/F Time, Hrs.

11,0 added to maintain approximately 50% P 0,

Following the procedure set forth above but placing a heating mantleover the surfaces of the reactor above the acid level produceddefluorinated acid of greater than /1 P/F ratio. Data are given in TableIII.

The series of runs were as follows:

1. 2 inch propeller X 1,750 RPM air rate approximately 126 liters in 20hours for 800 grams feed.

2. 3.4 inch propeller X 786 RPM same air rate as above.

3. 3.4 inch propeller X 786 RPM air rate equivalent to that 42 liters in20 hours for 800 gm. feed.

4. Same agitator same air rate, higher F in feed.

5. Same as No. 4 except used 1.5X amount of silica.

TABLE III Defluorination by Air Sparging with Heating Mantle Ten lbs. ofwet-process phosphoric acid having 53.3% P 0 0.64% F, 1.08% Al,O 1.61%Fe O 2.00% SO, and 1.57% solids was put in a 4-liter container equippedwith two baffles and a heating mantle covering the top of the containerand exhaust system. Air was introduced under a four-bladed turbineimpeller of 1.5 inch diameter revolving at 52.4 ft/sec tip speed.Diatomaceous earth having about 25 sq.m.lg. surface area was added tothe acid in the amount of 1 gm. of diatomaceous earth per gm. offluorine in the acid. When the temperature of the acid was 85 C. P/Fweight ratio of somewhat greater than 100 was achieved in a period ofabout hours.

Above experiment was repeated for 90 C. and 95 C. temperatures of theacid and respective defluorination times to reach P/F=100 were about 3hours and 1.8 hours.

EXAMPLE 9 Effects of Air Rate and Tip-Speed of the Impeller onDefluorination 1n the following tests 2.7 liters of wet processphosphoric acid, sp.g. 1.69, was placed in a 4-liter flask equipped with(1) an impeller driven by a variable speed motor, (2) an air sparger and(3) a heating mantle. The acid analyzed 53.3% P O 0.64% F, 1.08% Al O1.61% Fe O 2.00% S0 and contained 1.57 percent solids. The acid washeated in all tests to 85 C. and maintained at this temperature duringtreatments. Results obtained are set forth in Table IV below.

TABLE IV.

Air Rate Tip Speed Time in hours Cu. ftJmin/ton acid ft./sec. to reachP/F=O EXAMPLE l0 Defluorination experiments on larger scale were carriedout using 400 to 500 gallons of wet-process acid. The agitator was afour-bladed turbine type with one horsepower motor. Diatomaceous earthwas added in the amount of 1 lb. per lb. of fluorine in the acid. Allexperiments were carried out at 85 C. The agitator tip speed was ft/sec.The results are set forth in Table V below.

Effect of Different Types of Silicon-Bearing Materials on DefluorinationRates Different commercially available materials containing silica weretried for defluorination. The following table contains results ofdefluorination experiments and characteristics of different silicamaterials. All the experiments were carried out at C. using 10 lbs. ofwet process acid. The air volume used for sparging was 0.5liters/minute. The amount of silica material added was equivalent to 1gm. SiO per gm. of fluorine in the acid. The results obtained are setforth in Table VI below.

TABLE VI Effect of Type of Silica Material on Defluorination of WetProcess l Surface Area Dried Final Elapsed Time Silica CharacteristicsX: F F to Reach Material of the Material in the in the FinalDiatomaceous Acid Acid fluorine hours Earth 2143 sq.m.lgm.

Kenite 51 by Kenite Corp. 0.64 0.3 3.5

Diatomaceous Earth MN-35 40 sq.m.Igm. 0.64 0.2 6.0

by Johns-Manville Corp.

Silica Gel 340 sq.m.lgm. 0.64 0.3 4.0

Pore V01. =2.4 0.64 0.2 5.0

Silica Gel 500 sq.m.lgm. 0.64 0.3 5.0

Pore V01. =1 .0 0.64 0.2 6.0

Ground Sand 1.3 sq.m.lgm. 0.64 0.56 7.0

Sodium Silicate Na SiO,-91-1,0 0.78 0.73 5.0

Talc 1.6 sq.m.lgm. 0.64 0.52 4.0

Amorphous Silica 4.3 sq.m/gm. 0.64 0.60 10.0

From the above, it is evident (a) that diatomaceous silica andspray-dried silica gel are highly effective for defluorinating wetprocess acid when used in the process of the present invention and (b)that ground sand, sodium silicate and talc are only very poorlyeffective in this process.

EXAMPLE 12 In the following tests wet process phosphoric acid havingfrom 0.64 percent to 1.33 percent fluorine, 1.08% to 2.66% A1 0 and from48.2% to 54.52% P 0 was treated with a stoichiometric amount of silicicmaterial selected from the group consisting of diatomaceous silica,amorphous silica, ground sand and sodium silicate. The mixtures wereheated to about 85 C. and treated with (1) air sparged into the reactorbeneath the acid surface, (2) air passed through the acid in a packedcolumn or (3) by recirculating the acid from the reactor and spraying itas time droplets over the surface of the acid in the reactor. The dataobtained are given in Table VII and show that diatomaceous silica havinga surface area of 10 sq.m.lgm. used with air rates of 0.89 to 3.5 cu.ft./min. per ton of acid is effective for defluorinating acid maintainedat 85 C. These data also show that silica having a surface area of lessthan 10 sq.m.lgm. is ineffective for defluorinating phosphoric acid whenused under the above stated conditions.

TABLE VII Recircu- Surface Time to lation, Acid Starting acid, percentarea Air rate, Tip reach gal/nun.

wt, $102, cu. ft./min. Temp, speed, P/F=100 Product spray, Run No lbs. FA120: P205 S102. 1/b./lb.F nil/gm. per ton acid ft./sec hrs. P20 5nozzle 0. G4 1.08 53. 3 Diatomaceous... 10 3.5 85 13. 1 5

0. 64 V 2.16 53. 3 3. 5 85 52. 4 ll. 0

0. 33 53. 9 Amorphous silica. 4.3 3.5

O. 64 1.08 53. 9 Ground sand 1. 3 3. 5

0. 97 1. 33 53. 4 Diatomaceous 0. 8!)

l 8%lmln. of total acid. 2 12.5%/min. of total acid.

We claim:

1. A process for the formation of phosphoric acid having a P/F weightratio equal to or greater than 100 from wet process phosphoric acidcontaining an undesirable amount of fluorine as aluminum fluoridecomplexes, from about 0.8 percent by weight to about 2.2 percent byweight of dissolved aluminum and a P 0,, percent by weight of from about45 percent to about 62 percent comprising the steps of a. admixing saidwet process acid with an amount of silica at least aboutstoichiometrically equivalent to the fluorine to be removed therefrom;

. heating the mixture at a temperature in the range of from about 75C.to about 105C. but below the boiling point of said mixture;

. volatilizing the SiF. from the heated mixture into an inert gaseffluent stream by intimately contacting said heated mixture with aninert gas wherein the gas-mixture interface is sufficient to establish astripping ratio of the SiF from about 0.5 to about (1. maintaining thetemperature of said effluent stream above its dew point; and

e. removing said effluent stream from the presence of the acid mixture.

2. A process according to claim 1" wherein volatilizing the SiF iseffected by acid spraying employingabout 2 to 16 cubic feet of air perminute per ton of heated mixture.

3. A process according to claim 2 wherein the silica is selected fromthe group consisting of diatomaceous silica having a surface area offrom about to about 30 sq.m.lgram and spray dried silica gel having asurface area of from about 320 to about 500 sq.m.lgram.

4. A process according to claim 1 wherein the volatilizing of the SiF,is effected by air sparging employing from about 0.5 to about 10 cubicfeet of air per minute per ton of heated mixture.

5. A process according to claim 1 wherein the volatilizing of the SiF.is effected by air sparging employing from about 1 to about 5 cubic feetof air per minute per ton of heated mixture.

6. A process according to claim 5 wherein about 1.1 pound of silica isemployed per pound of fluorine contained in said heated mixture.

7. A process according to claim 6 wherein the silica is selected fromthe group consisting of diatomaceous silica or spray dried silica gelwherein the surface area of the silica is in the range of between 10 and40 sq.m.lgram.

8. A process according to claim 1 wherein the volatilizing of the SiF.is effected by a combination of acid spraying and air sparging.

9. A process according to claim 1 wherein the process is continuous.

10. A process for the defluorination of wet process phosphoric acidcontaining from about 0.3 percent to about 3 percent by weight offluorine as aluminum fluoride complexes, from about 0.8 percent byweight to about 2.2 percent by weight of dissolved aluminum and a P 0percent by weight of from about 45 percent to about 62 percentcomprising the steps of:

a. admixing said wet process acid with an amount of silica at leastabout stoichiometrically equivalent to the fluorine to be removedtherefrom;

b. heating the mixture at a temperature in the range of from about C. toabout C. but below the boiling point of said mixture;

. volatilizing the SiF from the heated mixture into an inert gaseffluent stream by intimately contacting said heated mixture with aninert gas wherein the gas-mixture interface is sufficient to establish astripping ratio of the SiF above about 1.5 and maintain the dissolvedsilicon concentration below about 0.05 percent by weight weightsubstantially throughout the defluorination;

d. maintaining the temperature of said effluent stream above its dewpoint; and

e. removing said effluent stream from the presence of the acid mixture.

11. A process according to claim 10 wherein the F/Si weight ratio ismaintained above about 8 substantially throughout the defluorination.

2. A process according to claim 1 wherein volatilizing the SiF4 iseffected by acid spraying employing about 2 to 16 cubic feet of air perminute per ton of heated mixture.
 3. A process according to claim 2wherein the silica is selected from the group consisting of diatomaceoussilica having a surface area of from about 15 to about 30 sq.m./gram andspray dried silica gel having a surface area of from about 320 to about500 sq.m./gram.
 4. A process according to claim 1 wherein thevolatilizing of the SiF4 is effected by air sparging employing fromabout 0.5 to about 10 cubic feet of air per minute per ton of heatedmixture.
 5. A process according to claim 1 wherein the volatilizing ofthe SiF4 is effected by air sparging employing from about 1 to about 5cubic feet of air per minute per ton of heated mixture.
 6. A processaccording to claim 5 wherein about 1.1 pound of silica is employed perpound of fluorine contained in said heated mixture.
 7. A processaccording to claim 6 wherein the silica is selected from the groupconsisting of diatomaceous silica or spray dried silica gel wherein thesurface area of the silica is in the range of between 10 and 40sq.m./gram.
 8. A process according to claim 1 wherein the volatilizingof the SiF4 is effected by a combination of acid spraying and airsparging.
 9. A process according to claim 1 wherein the process iscontinuous.
 10. A process for the defluorination of wet processphosphoric acid containing from about 0.3 percent to about 3 percent byweight of fluorine as aluminum fluoride complexes, from about 0.8percent by weight to about 2.2 percent by weight of dissolved aluminumand a P2O5 percent by weight of from about 45 percent to about 62percent comprising the steps of: a. admixing said wet process acid withan amount of silica at least about stoichiometrically equivalent to thefluorine to be removed therefrom; b. heating the mixture at atemperature in the range of from about 75* C. to about 105* C. but belowthe boiling point of said mixture; c. volatilizing the SiF4 from theheated mixture into an inert gas effluenT stream by intimatelycontacting said heated mixture with an inert gas wherein the gas-mixtureinterface is sufficient to establish a stripping ratio of the SiF4 aboveabout 1.5 and maintain the dissolved silicon concentration below about0.05 percent by weight weight substantially throughout thedefluorination; d. maintaining the temperature of said effluent streamabove its dew point; and e. removing said effluent stream from thepresence of the acid mixture.
 11. A process according to claim 10wherein the F/Si weight ratio is maintained above about 8 substantiallythroughout the defluorination.